The earlier Zelda games were tough, but ask most people on the street what the toughest dungeon in the entire series is and they'll probably say it's Ocarina of Time's Water Temple. Even over a decade on, it still hurts to say it out loud.
I never thought it was that hard (just a little disorientating), but hey, to each their own. Anyway, this video is great, and would have been very handy at the time: it's Zelda director Eiji Aonuma, the man who designed the dungeon (and Ocarina's other dungeons as well), taking you through its trickiest parts.
Probably not of much use to most of you, but think of it as a director's commentary of sorts and it's fun viewing.